File sharing device and inter-file sharing device data migration method

ABSTRACT

Data migration from an existing NAS to a new NAS and processing of access from a client are executed in parallel. A migration destination NAS inherits the host name and IP address of a migration source NAS. The network environment parameters of the migration source NAS are changed to different values. The migration destination NAS causes data to migrate from a migration source storage region, in which a shared file system of the migration source NAS is mounted, to a migration destination storage region. In a case where there is an access request from the client during data migration, the migration destination NAS selects one of the storage region in correspondence to the migration status of the requested data and provides that data. Data of a “not copied” status is provided from the migration source storage region. Data of a “copied” status is provided from the migration destination storage region.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.10/767,442 filed on Jan, 30, 2004. Priority is claimed based on U.S.application Ser. No. 10/767,442 filed on Jan. 30, 2004, which claimspriority to Japanese Patent Application No. 2003-316291 filed on Sep. 9,2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a file sharing device known as a fileserver or NAS (Network Attached Storage) and an inter-file sharingdevice data migration method.

2. Description of the Related Art

File servers are used in order for data to be shared between pluralcomputer terminals dispersed on a network. As an early file server, aserver where a file sharing protocol such as CIFS (Common Internet FileSystem) or NFS (Network File System) is disposed in a versatile OS(Operating System) is known. As an improved file server, an NAS where adedicated OS specializing in file sharing services is used to supportplural file sharing protocols (CIFS, NFS, DAFS (Direct Access FileSystem), etc.) is known.

The number of files to be shared and the number of clients using sharedfiles have been increasing each year, and individual file sizes alsocontinue to increase. In order to accommodate the increased demand forfile sharing services, storage devices and communication lines are beingstrengthened and technology is switching over to high-performance fileservers.

Several methods of causing data to migrate from old file servers to newfile servers are known. A first method is a method that temporarilysuspends file access from a client and occupies the migration sourcefile server (old file server) and the migration destination file server(new file server) to carry out data copying between both file servers. Asecond method is a method that backs up data retained in the migrationsource file server and causes the migration destination file server torestore the backup data.

A third method that causes data to migrate without stopping the filesharing service is also known (JP-A-2000-173279). In this third method,when access is made by a client during data migration and data to whichaccess has been requested is data that has already been copied to themigration destination file server, the data is provided from themigration destination file server to the client. Alternatively, when thedata to which access has been requested is data that has not yet beencopied to the migration destination file server, the data is providedfrom the migration destination file server to the client once the datahas been copied from the migration source file server to the migrationdestination file server.

In the first method, the client cannot use the file sharing serviceuntil data migration is completed, because the migration source fileserver and the migration destination file server are occupied during thedata migration. In a case where the amount of migration target data isrelatively small, it is possible to resume the file sharing service fora short period. However, as the amount of data increases, the stoppedtime of the file sharing service increases.

In the second method, the file sharing service can be continued usingthe migration source file server while the backup data is being restoredin the migration destination file server. However, when the clientupdates the data during restoration, a difference arises between thelatest data of the migration source file server and the backup data.Thus, for example, an access log during restoration is kept and, afterrestoration is completed, the storage content of the migrationdestination file server is updated to the latest state on the basis ofthe access log. In this case, after restoration is completed, labor forsynchronizing the storage contents between the migration destinationfile server and the migration source file server becomes necessary, andthis takes time. As another method, providing a file sharing service ina read-only mode during restoration is also conceivable. However, thisis not very convenient because the client can only read the data.

In the third method, the file sharing service can be provided to theclient even during data migration. However, in the third method, in acase where the client requests access to data that has not yet beencopied to the migration destination file server, the data is copied fromthe migration source file server to the migration destination fileserver and provided from the migration destination file server to theclient. In other words, in the third method, the responsiveness of theservice drops because the data to which access has been requested by theclient is provided to the client after it is first copied to themigration destination file server. Also, in the third method, theconfiguration becomes complicated and costs also rise because it isnecessary to dispose a data migration-dedicated interface in both of themigration source file server and the migration destination file server.

SUMMARY OF THE INVENTION

The present invention was made in light of the above-described problems,and it is an object thereof to provide a file sharing device and aninter-file sharing device data migration method that can conduct datamigration without stopping a file sharing service.

It is another object of the invention to provide a file sharing deviceand an inter-file sharing device data migration method that can providea file sharing service with high responsiveness during data migration.

It is yet another object of the invention to provide a file sharingdevice and an inter-file sharing device data migration method that canindependently execute, in parallel, data migration processing andprocessing of an access request from a host computer.

It is still another object of the invention to provide a file sharingdevice and an inter-file sharing device data migration method configuredto control the priority of data to be migrated in accordance with theuse frequency of the data.

Further objects of the invention will become apparent from thedescription of the embodiment discussed later.

In order to achieve these objects, a file sharing device according tothe invention is communicably connected to a migration source filesharing device and a host computer via a communications network, andincludes data migrating means and access processing means. The datamigrating means causes data retained in a file system of the migrationsource file sharing device to migrate to a file system of the filesharing device. The access processing means processes access from thehost computer in correspondence to data migration statuses during thedata migration processing by the data migrating means. Additionally, theaccess processing means causes data to be provided from the file sharingsystem of the file sharing device in the case of access to data that canbe used from the file system of the file sharing device and providesdata from the file system of the migration source file sharing device inthe case of access to data that cannot be used from the file system ofthe file sharing device.

Here, examples of the file sharing device include an NAS or a fileserver where a file sharing protocol is disposed in a versatile OS. Thefile sharing device of the migration source and the file sharing device(referred to below as “the migration destination file sharing device”)are connected via a communications network such as a LAN (Local AreaNetwork) . Also, the file sharing devices and the host computer areconnected via the communications network. It should be noted that theinvention is not limited to a case where all of the file system that themigration source file sharing device has is caused to migrate to themigration destination file sharing devices and also includes a casewhere part of the file system is caused to migrate.

The data migrating means causes data stored in the file system of themigration source file sharing device (referred to below as “themigration source file system”) to migrate to the file system of themigration destination file sharing device (referred to below as “themigration destination file system”) In a case where there is a requestfor access to data by the host computer during data migration, theaccess processing means selects the provider of that data incorrespondence to data migration statuses. That is, the accessprocessing means causes data to be provided from the migrationdestination file system in a case where the request is a request foraccess to data that can be provided from the migration destination filesystem. The access processing means causes data to be provided from themigration source file system in a case where the request is a requestfor access to data that cannot be provided from the migrationdestination file system. Thus, access requests from the host computercan be received and processed while conducting data migration from themigration source file system to the migration destination file system,and data migration processing and access request processing can beexecuted in parallel.

In an aspect of the invention, the invention further includes connectionswitching means that switches access by the host computer from themigration source file sharing device to the migration destination filesharing device, wherein the connection switching means inherits networkenvironment information for identifying the migration source filesharing device on the communications network and updates, to othervalues, the network environment information that had been set in themigration source file sharing device.

The migration source file sharing device and the migration destinationfile sharing device are identified on the communications network bynetwork environment information. Examples of the network environmentinformation include computer names and identification information (IPaddresses, etc.). Depending on the specification of the communicationsnetwork, the information necessary for distinguishing the computerresources on the network will differ. In a protocol such as TCP/IP,unique identification information such as an IP address is required inaddition to a computer name. In other protocols, the computer resourceson the network are identified by the computer name only.

The connection switching means causes the migration destination filesharing device to inherit the network environment information that hadbeen set in the migration source file sharing device. The connectionswitching means also sets, to other values, the network environmentinformation that had been set in the migration source file sharingdevice. Only the migration destination file sharing device grasps thenew network environment information set in the migration source filesharing device. Thus, after the network environment information of themigration source file sharing device has been reset, only the migrationdestination file sharing device can access the migration source filesharing device.

More specifically, the connection switching means first sets the networkenvironment information of the migration source file sharing device toother values. Next, the connection switching means sets the networkenvironment information of the migration destination file sharing deviceto values that are the same as those of the original network environmentinformation of the migration destination file sharing device. Thus, themigration source file sharing device and the migration destination filesharing device are switched on the communications network. Therefore,access can be switched from the migration source file sharing device tothe migration destination file sharing device without the host computerperceiving this.

In an aspect of the invention, the invention further includes attributeinformation inheriting means that inherits attribute informationretained in the migration source file sharing device, wherein the datamigration processing by the data migrating means is initiated afterinheritance of the attribute information by the attribute informationinheriting means has been completed.

Examples of the attribute information include directory information andaccess management information. Because data migration processing isconducted after the migration destination file sharing device has beenmade to inherit the attribute information that the migration source filesharing device has, access processing corresponding to the attributes ofthe data can be conducted even if there is an access request from thehost computer during data migration processing.

In an aspect of the invention, the invention further includes usefrequency detecting means that detects the use frequency of data thatcannot be used from the migration destination file system, wherein thedata migrating means gives priority to and causes data to migratebeginning with data whose use frequency detected by the use frequencydetecting means is equal to or greater than a predetermined value.

That is, in a case where access to data whose migration from themigration source file system to the migration destination file systemhas not been concluded is requested by the host computer, the usefrequency detecting means measures the number of occurrences of accessrequests. Data whose use frequency is equal to or greater than thepredetermined value is data whose request for access from the hostcomputer is strong. The data migrating means gives priority to andcauses data to migrate beginning with data having a high use frequency.Thus, data is given priority and causes to migrate to the migrationdestination file system beginning with data having a high use frequency,and data can be provided from the migration destination file system in acase where there is new access from the host computer.

In an aspect of the invention, the data migration statuses include (A) afirst migration status representing a status where migration of datafrom the migration source file system to the migration destination filesystem has not been conducted, (B) a second migration statusrepresenting a status where data is migrating from the migration sourcefile system to the migration destination file system, (C) a thirdmigration status representing a status where migration of data from themigration source file system to the migration destination file system,and (D) a fourth migration status representing a status where data isbeing provided from the migration source file system; and, (a) in thecase of the first migration status, the access processing means changesthe migration status of data to which access has been requested to thefourth migration status, provides the data from the file system of themigration source file sharing device, and thereafter returns themigration status of the data to the first migration status, and (b) inthe case of the second migration status, the access processing meansprovides the data from the file system of the migration source filesharing device in a read-only mode, and (c) in the case of the thirdmigration status, the access processing means provides the data from thefile system of the migration destination file system, and (d) in thecase of the fourth migration status, the access processing meansprovides the data from the migration source file system in the read-onlymode and thereafter changes the migration status of the data to thefirst migration status.

That is, the method of providing data is switched in correspondence tothe four types of data migration statuses.

A method of data migration between file sharing devices according toanother perspective of the invention is a method of causing data tomigrate from a file system of a migration source file sharing device toa file system of a migration destination file sharing device, the methodincluding the steps of: causing the migration destination file sharingdevice to inherit, prior to data migration, attribute information thatthe migration source file sharing device has; causing access from a hostcomputer to be switched from the migration source file sharing device tothe migration destination file sharing device; detecting the migrationstatus of data to which access has been requested by the host computer;providing the data from the file system of the migration destinationfile sharing device in a case where the detected migration status of thedata is a status where the data can be used from the file system of themigration destination file sharing device; and providing the data fromthe file system of the migration source file sharing device in a casewhere the detected migration status of the data is a status where thedata cannot be used from the file system of the migration destinationfile sharing device.

A computer program according to another aspect of the invention is acomputer program that allows a computer to execute a method of causingdata to migrate from a migration source file sharing device, the methodincluding the steps of: causing the computer to inherit, prior to datamigration, attribute information that the migration source file sharingdevice has; causing access from a host computer to be switched from themigration source file sharing device to the computer; detecting themigration status of data to which access has been requested by the hostcomputer; providing the data from the file system of the computer in acase where the detected migration status of the data is a status wherethe data can be used from a file system of the computer; and providingthe data from a file system of the migration source file sharing devicein a case where the detected migration status of the data is a statuswhere the data cannot be used from the file system of the computer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the overall configuration of a filesharing system according to an embodiment of the invention;

FIG. 2 is an explanatory diagram showing the functions that a migrationdestination NAS has;

FIG. 3 is an explanatory diagram showing an example of migrationsource/migration destination file system configuration information;

FIG. 4A is an explanatory diagram showing an example of migration sourcesetting change information, and FIG. 4B is an explanatory diagramshowing an example of temporary setting information used in thetemporary launching of the migration destination NAS;

FIG. 5 is a flow chart schematically showing the overall operation of adata migration method;

FIG. 6 is a flow chart showing data migration processing and clientaccess processing;

FIG. 7 is a flow chart showing the details of client access controlprocessing of FIG. 6;

FIG. 8 is a continuation of the flow chart of FIG. 7;

FIG. 9 is another continuation of the flow chart of FIG. 7;

FIG. 10 is yet another continuation of the flow chart of FIG. 7;

FIG. 11 is an explanatory diagram schematically showing the switching ofnetwork environment information of a migration source NAS and themigration destination NAS;

FIG. 12 is an explanatory diagram schematically showing a conditionwhere client access arising during data migration processing isprocessed; and

FIG. 13 is a schematic diagram showing a state where a file sharingsystem provided to the client is selected in accordance with the statusof data migration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention will be described below on the basis ofFIGS. 1 through 13.

In the present invention, as will be described below, a storage region(file system) of a migration source file server serving as a migrationtarget is occupied by a migration destination file server when datamigration is carried out between the migration source file server andthe migration destination file server. At this time, the migrationdestination file server inherits network environment information (e.g.,computer name and IP address) of the migration source file server. Also,the network environment information of the migration source file serveris updated to new values that only a migration destination file sharingdevice recognizes. The inheriting of the network environment informationand the setting of the new network environment information are carriedout by remote control from the migration destination file server. Thus,in a case where the client of a file sharing service conductsconventional access, the client is connected to the migration sourcefile server without being cognizant of this.

In this state, the migration destination file server copies, to its ownfile system, data stored in the file system of the migration source filesystem. At the time of data migration, the migration destination fileserver manages the status of data copying with four types of statuses:“not copied”, “being copied”, “copied”, and “accessing migrationsource”. In a case where an access request from a client arises duringdata migration, the migration destination file server checks the statusof the data to which access has been requested. The migrationdestination file server determines the provider of the data and providesthe data to the client in correspondence to the migration status of thedata. That is, the migration destination file server provides the clientwith the data copied to its own file system, or provides the client withdata from the file system of the migration source file server that themigration destination file server is occupying.

Thus, in the present embodiment, data migration between file servers andan access request from the client can be independently processed inparallel, and a file sharing service with good responsiveness can beprovided even during data migration processing.

FIG. 1 is a block diagram showing the overall configuration of the filesharing system. In this system, plural client computers 10(1) to 10(n),a migration source NAS 20 and a migration destination NAS 30 areinterconnected via a communications network CN such as a LAN. It shouldbe noted that, in the following description, “client computer” will beabbreviated as “client” and the reference numeral “10” will be used whenno particular distinction is to be made between the clients.

Each client 10 is configured as a computer device, such as a personalcomputer, a work station or a portable information terminal. Applicationprograms that use a file sharing service are respectively disposed inthe clients 10. Each client 10 is connected to the communicationsnetwork CN via a network interface card (NIC). For example, whenhigh-speed data communication is necessary, an NIC with hardwareacceleration that shoulders TCP/IP processing is used. Each client 10can be dispersed and disposed in respectively different locations or canbe disposed in the same location.

The migration source NAS (old NAS) 20 can be configured as asophisticated file server specializing in file sharing services. Itshould be noted that the migration source NAS 20 may also be configuredas an ordinary file server where a file sharing protocol is disposed ona versatile OS. The migration source NAS 20 is disposed with a migrationsource shared file system 21, a communication unit 22 and a storage unit23.

The migration source shared file system 21 stores, in a hierarchy,numerous files shared by the clients 10. For example, a storage regionis formed by connecting a storage device such as a hard disk drive or asemiconductor memory device with a SAN (Storage Area Network). Theshared file system 21 uses the logical storage region formed on thephysical storage region to store data in file units.

As described above, the communication unit 22 can be configured as anNIC with hardware acceleration. The migration source NAS 20 is connectedto the communications network CN via the communication unit 22. Themigration source NAS 20 provides the file sharing service to the clients10 via the communications network CN from the communication unit 22.However, as described later, during data migration, the migration sourceNAS 20 is occupied by the migration destination NAS 30 so that theclients 10 cannot directly access the migration source NAS 20.

The storage unit 23 is configured as a nonvolatile semiconductor memorydevice. User information 231 and network environment information 232 arestored in the storage unit 23. User accounts and access rights areincluded in the user information 231. Each client 10 can access thefiles stored in the shared file system 21 on the basis of preset accessrights. Whether or not the access requests from the clients 10 arepermitted is determined on the basis of the user information 231.Information necessary to identify the migration source NAS 20 on thenetwork is included in the network environment information 232.

Among the information necessary for identification are computer names(host names, etc.) and unique identification information (IP addresses,etc.). The types of information necessary for identifying respectivenodes participating in the network are dependent on the communicationprotocol used in that network. For example, in the case of TCP/IP, thehost name and unique identification information are necessary toidentify the node. In a case where the specification of thecommunication protocol can be identified simply by the computer name,such as in NETBEUI (NetBIOS Extended User Interface) and IPX/SPX(Internetwork Packet Exchange/Sequenced Packet Exchange), the networkenvironment information 232 may be configured to include only thecomputer name.

Similar to the migration source NAS 20, the migration destination NAS 30can be configured as an ordinary file server using a versatile OS or asa sophisticated NAS. The migration destination NAS 30 is disposed with acommunication unit 31, a control unit 32, a migration source storageregion 33 and a migration destination storage region 34.

The communication unit 31 is configured in the same manner as thecommunication unit 22. The migration destination NAS 30 is connected,through the communication unit 31, to the clients 10 and the migrationsource NAS 20 via the communications network CN.

The control unit 32 controls the entire operation of the migrationdestination NAS 30. The control unit 32 can be configured to include atleast one CPU (Central Processing Unit). The control unit 32 is disposedwith a function 40 of alternating new and old servers (referred to belowas “new/old alternating function”), a data migration control function 50and a management table 60. The new/old alternating function 40 logicallyalternates the migration source NAS 20 and the migration destination NAS30 on the network. That is, the clients 10 can connect to the migrationsource NAS 20 with a conventional connection setting, but the actualconnection destination is the migration destination NAS 30.

The data migration control function 50 respectively executes, inparallel, processing of data migration from the migration source NAS 20to the migration destination NAS 30 and processing of access requestsfrom the clients 10 arising during data migration processing. The datamigration control function 50 controls the data provider incorrespondence to the migration status of the data. The management table60 manages the statuses of the files during data migration. The detailsof the control unit 32 and the management table 60 will be discussedlater.

The migration source storage region 33 is a virtual storage region thatis disposed in order for data retained in the migration source sharedfile system 21 to be used during data migration processing. A virtualmigration source shared file system 331, which is constructed bynetwork-mounting the migration source shared file system 21, is disposedin the migration source storage region 33. The migration source storageregion 33 occupies the network-mounted migration source shared filesystem 21. That is, during data migration processing, data of themigration source shared file system 21 can be accessed only through themigration destination NAS 30. It should be noted that the migrationsource storage region 33 becomes unnecessary after completion of thedata migration processing.

The migration destination storage region 34 is disposed with a migrationdestination file system 341. The migration destination storage region 34is a storage region for providing the clients 10 with the data that hasmigrated from the migration source shared file system 21. The status ofdata migration from the migration source storage region 33 to themigration destination storage region 34 is monitored per piece of data(file), and the control unit 32 is notified of this status.

The primary functions of the control unit 32 will now be described withreference to FIG. 2. The new/old alternating function 40 is disposedwith an existing environment setting changing function 41 and amigration source information inheriting function 42 as primaryfunctions. A remote control function 43 is a function that is used whenthe existing environment setting changing function 41 and the migrationsource information inheriting function 42 are executed. A userinformation inheriting function 44 and a network environment informationinheriting function 45 are sub-functions of the migration sourceinformation inheriting function 42. A migration source restartmonitoring function 46 is a sub-function of the existing environmentsetting changing function 41.

The existing environment setting changing function 41 sets the migrationsource NAS 20 to an occupiable state by remote control (e.g.,communication using an r-command or a remote terminal protocol such asTELNET) through the communications network CN. The existing environmentsetting changing function 41 rewrites the network environmentinformation of the migration source NAS 20 over other preparedinformation, whereby the existence of the migration source NAS 20 ishidden from the clients 10. Thus, the migration source NAS 20 becomesaccessible only by the migration destination NAS 30. As a result, themigration source NAS 20 is occupied by the migration destination NAS 30.

The migration source information inheriting function 42 causes themigration destination NAS 30 to inherit the user information 231 and thenetwork environment information 232 that the migration source NAS 20manages. The user information inheriting function 44 causes themigration destination NAS 30 to inherit the user accounts and accessrights that the migration source NAS 20 manages. By inheriting the userinformation, access rights management that is the same as before datamigration can be conducted even after data migration. The networkenvironment information inheriting function 44 inherits the networkenvironment information that had been set in the migration source NAS20. Because the migration destination NAS 30 inherits the networkenvironment information of the migration source NAS 20, the clients 10perceive the migration destination NAS 30 as the migration source NAS20.

The migration source restart monitoring function 46 monitors whether ornot the migration source NAS 20 has restarted in accordance with the newnetwork environment information set by the existing environment settingchanging function 41.

The data migration control function 50 is disposed with a migrationsource/migration destination configuration managing function (referredto below as “the configuration managing function”) 51, a data migrationprocessing function 52, a data copying function 53, a copystatus-managing function 54 and a client access control function 55. Theconfiguration managing function 51 is a function that manages the filesystem configuration of the data region of the migration source and thefile system configuration of the data region of the migrationdestination. The data migration processing function 52 is a functionthat executes, in parallel, data migration processing and processing ofclient access arising during data migration processing.

The data copying function 53, the copy status managing function 54 andthe client access control function 55 are sub-functions of the datamigration processing function 52. The data copying function 53 copies,to the migration destination shared file system 341, data stored in themigration source shared file system 21. In the case of the presentembodiment, copying is executed in file units. However, depending on thecase, copying may also be executed in block units. The copy statusmanaging function 54 manages the copy status of the data-i.e., themigration status of the migration target data (files) . The clientaccess control function 55 is a function that determines the source ofdata provided to the client 10 when a file access request from theclient 10 arises during data migration. The client access controlfunction 55 provides, to the client 10, the requested data from one ofthe migration source storage region 33 and the migration destinationstorage region 34 on the basis of the data migration status managed bythe copy status managing function 54.

Next, the various types of information managed by the management table60 will be described on the basis of FIG. 3 and FIGS. 4A and 4B. In thepresent embodiment, an example of a case using TCP/IP will be described.First, FIG. 3 shows migration source/migration destination file systemconfiguration information (referred to below as “file systemconfiguration information”).

In the file system configuration information, the name of the migrationsource NAS 20 (migration source host name), the migration targetmigration source shared file system name, the mount point when themigration source shared file system is mounted in the migrationdestination NAS 30, and the name of the migration destination sharedfile system storing the migration target data are corresponded. Also,the data migration status of each file included in the shared filesystem is managed in file units per migration target migration sourceshared file system. That is, the file name, the migration status of thatfile and its not-copied use frequency are corresponded.

The four types of “not copied”, “being copied”, “copied” and “accessingmigration source” are prepared as the data migration statuses, and eachpiece of the migration target data has one status. Here, the “notcopied” status refers to a status where copying of the data from themigration source storage region 33 to the migration destination storageregion 34 has not been conducted. The “being copied” status refers to astatus where copying of the data currently from the migration sourcestorage region 33 to the migration destination storage region 34 iscurrently being conducted. The “copied” status refers to a status wherecopying of the data from the migration source storage region 33 to themigration destination storage region 34 has been completed. The“accessing migration source” status refers to a status where the datastored in the migration source storage region 33 is being used forclient access because copying of the data from the migration sourcestorage region 33 to the migration destination storage region 34 has notyet been executed. That is, the “accessing migration source” status is astatus where the client 10 is accessing the migration source storageregion 33 selected as the data source.

The “not-copied use frequency” is a parameter for detecting usefrequency with respect to data that has not yet migrated from themigration source storage region 33 to the migration destination storageregion 34. The “not-copied use frequency” represents migration demandwith respect to data that has not yet migrated.

In the file system configuration information, the content of theparameter of the migration source shared file system 21 and the contentof the parameter of the migration destination shared file system 341 canbe set to be the same. In this case, each client 10 can use, as is, theconventionally used setting of the shared file system when accessing themigration destination NAS 30. In other words, it is possible for eachclient 10 to access data in the same manner as had been the case untilnow, without being cognizant of the fact that the shared file system hasbeen switched. Conversely, the content of the parameter of the migrationsource shared file system 21 and the content of the parameter of themigration destination shared file system 341 can also be set to bedifferent. In this case, the data can be re-disposed while the data ismigrating from the migration source storage region 33 to the migrationdestination storage region 34. It should be noted that, in a case wherethe parameters of the shared file systems are made different, theclients 10 are made aware of the configuration (mount name, etc.) of theshared file system 341 that the migration destination NAS 30 has whenthe clients 10 access the migration destination NAS 30.

In FIG. 1, one migration source storage region 33 and one migrationdestination storage region 34 are shown. However, in a case where themigration source NAS 20 supports different types of file sharingprotocols such as NFS (Network File System; “NFS” is a registeredtrademark of Sun Microsystems, Inc.) and CIFS (Common Internet FileSystem), migration source storage regions and migration destinationstorage regions are respectively prepared for each file sharingprotocol. For convenience of explanation, a case will be described wherethe clients 10 use the same file sharing protocol to utilize the filesharing service in FIG. 1. However, it goes without saying that it ispossible for plural types of file sharing protocols to be supported inthe present embodiment.

Next, FIG. 4A shows migration source setting change information forswitching the network environment information of the migration sourceNAS. Network environment information of the migration source NAS 20prior to data migration (initial network environment information) andnetwork environment information to be switched at the time of datamigration (change-use network environment information) are included inthe migration source setting change information. The initial name priorto the change (server name), the initial IP address (including a subnetmask), the administrator user account name and the migration sourceshared file system name that the migration source NAS 20 has areincluded in the initial network environment information. The host nameafter the change, the IP address after the change (including the subnetmask) and the administrator user account name after the change areincluded in the change-use network environment information.

The client 10 identifies the access destination on the basis of the hostname and the IP address. As shown in the migration source setting changeinformation, the migration source NAS 20 becomes hidden from the networkby changing the host name and IP address of the migration source NAS 20data migration. The changed host name and IP address are grasped only bythe migration destination NAS 30. Thus, the only device that canactually directly access the migration source NAS 20 is the migrationdestination NAS 30, and the migration source NAS 20 is occupied by themigration destination NAS 30.

FIG. 4B shows temporary setting information of the migration destinationNAS 30 prior to data migration. The migration destination NAS 30inherits the network environment information of the migration source NAS20 during data migration, whereby the migration destination NAS 30 isdisguised with respect to the clients 10 as if it were the migrationsource NAS 20. Before the migration destination NAS 30 inherits thenetwork environment information of the migration source NAS 20, it isnecessary for other network environment information different from thisto be set in the migration destination NAS 30. Temporary networkenvironment information used in order to make a temporary launch at apreliminary stage of data migration is preregistered in the temporarysetting information. A temporary host name, a temporary IP address and atemporary subnet mask are included in the temporary network environmentinformation.

It should be noted that host names and IP addresses are included in themigration source setting change information and the temporary settinginformation, but these are dependent on the communication protocol thatis used (in the present embodiment, TCP/IP). Thus, depending on thespecification of the communication protocol, in a case where just thehost name is sufficient, it is not necessary to set uniqueidentification information for clearly identifying the device on thenetwork, such as an IP address.

Due to the above configuration, the clients 10 that had been accessingthe migration source NAS 20 and using the file sharing service canaccess the migration destination NAS 30 and continue to use the filesharing service without making special setting changes during and afterdata migration.

In a case where an access request from the clients 10 arises while thedata is migrating from the migration source NAS 20 to the migrationdestination NAS 30, the client access control function 55 determines themigration status of the requested data. In a case where the datarequested by the clients 10 has already been copied to the migrationdestination storage region 34 and can be used, the client access controlfunction 55 provides the data stored in the migration destinationstorage region 34 to the clients 10. In a case where the data requestedby the clients 10 has not yet been copied to the migration destinationstorage region 34, the client access control function 55 provides thedata stored in the migration source storage region 33 to the clients 10.Thus, each client 10 can obtain the desired data simply by accessing themigration destination NAS 30, even in a case where data migration hasbeen completed or in a case where data migration has not been completed.

FIG. 5 is a flow chart showing the overall flow of the method by whichdata migrates between the NAS 20 and the NAS 30. First, the migrationdestination NAS 30 starts on the basis of the temporary networkenvironment information shown in FIG. 4B (S1). Thus, preparation forinheriting the network environment information from the migration sourceNAS 20 is completed. The migration destination NAS 30 is recognized onthe network as a device having the host name “FS1 target” and the IPaddress “192.168.100.10”. Next, the data migration configuration of themigration source NAS 20 and the migration destination NAS 30 andinformation necessary for carrying out data migration such as the systemenvironment information are registered (S2). Next, access by the clients10 accessing the migration source NAS 20 is temporarily suspended (S3).

The migration destination NAS 30 acquires, by remote control via thecommunications network CN, the user information 231 and the networkenvironment information 232 that are to be inherited from the migrationsource NAS 20 (S4). The migration source NAS 20 responds with respect tothe information acquisition request from the migration destination NAS30 (S5).

Next, the migration destination NAS 30 changes, by remote control viathe communications network CN, the network environment information ofthe migration source NAS 20 to the change-use network environmentinformation shown in FIG. 4A, and restarts the migration source NAS 20using a reboot command (S6). In the present embodiment, a case isdescribed where only the network environment information of themigration source NAS 20 is changed, but the user information of themigration source NAS 20 may also be changed as needed. The networkenvironment information of the migration source NAS 20 is changed fromthe initial network environment information to the change-use networkenvironment information and the migration source NAS 20 is restarted,whereby the migration destination NAS 30 can occupy the migration sourceNAS 20.

The migration source NAS 20 stops the system in accordance with therestart request from the migration destination NAS 30 (S7), and restartson the basis of the designated network environment information (S8).When the system of the migration source NAS 20 is restarted (S9), themigration source NAS 20 responds with respect to the restart monitoringfrom the migration destination NAS 30 (S10, S11).

In other words, after the migration destination NAS 30 instructs themigration source NAS 20 to restart (S6), it monitors whether or not themigration source NAS 20 has normally restarted on the basis of thedesignated network environment information (S10). This monitoring can beconducted by the migration destination NAS 30 continually calling, inrelatively short cycles, on the host name and IP address designated bythe change-use network environment information.

When the migration destination NAS 30 confirms that the migration sourceNAS 20 has restarted, the user information and the network environmentinformation (initial network environment information) acquired from themigration source NAS 20 is reflected in the migration destination NAS 30(S12). The network environment information of the migration destinationNAS 30 is changed to the network environment information of themigration source NAS 20, whereby the migration destination NAS 30 isperceived on the communications network CN as the migration source NAS20.

Due to the above processing, preparation for initiating data migrationis completed. The migration destination NAS 30 network-mounts the sharedfile system 21 of the migration source NAS 20 and occupies the migrationsource NAS 20. The migration destination NAS 30 copies and migrates thedata of the mounted migration source shared file system 331 to themigration destination shared file system 341 (S13, S14).

Even during data migration, the file sharing service is provided to theclients 10 without being stopped. The clients 10 can request access todata without being cognizant of whether the desired data is located inthe migration source storage region 33 or the migration destinationstorage region 34. The clients 10 can also request access to datawithout being cognizant of the migration status of the desired data. Ina case where an access request from the client 10 arises during datamigration (S15), the migration destination NAS 30 selects one of thestorage regions 33 and 34 in correspondence to the data migration statusand provides the client 10 with the requested data. That is, themigration destination NAS 30 independently executes, in parallel, thedata migration processing and the processing of access from the client10 (S13) . The details of S13 will be described later.

Additionally, in a case where data migration from the migration sourceNAS 20 to the migration destination NAS 30 has been completed, thesystem administrator is notified that data migration has been completed(S16). This notification can be conducted using, for example, electronicmail. After the completion of data migration, the migration source NAS20 becomes unnecessary because the migration destination NAS 30 providesthe file sharing service. The migration source NAS 20 can be used forother services. Alternatively, the migration source NAS 20 may be usedas a backup server.

FIG. 6 is a flow chart showing the details of S13 in FIG. 5. First, themigration destination NAS 30 mounts the migration source shared filesystem 21 at a preset migration destination mount point (S21). Next, themigration destination NAS 30 makes the migration destination shared filesystem 341 network-accessible (S22). Thus, the clients 10 can access thestorage region of the migration destination NAS 30 via thecommunications network CN.

Additionally, below-described data migration processing (S25 to S32) andclient access processing (S23, S24.) are independently conducted inparallel. First, attention will be given to the client accessprocessing. The migration destination NAS 30 monitors whether or not anaccess request from the clients 10 has arisen during data migration(S23). In a case where an access request from the clients 10 has arisen,the migration destination NAS 30 provides the clients 10 with the datain correspondence to the migration status of the requested data (S24).The details of the client access control processing shown in S24 will bedescribed later.

Turning now to the data migration processing, the migration destinationNAS 30 checks the file system configuration information shown in FIG. 3(S25). The migration destination NAS 30 determines whether or not dataexceeding a predetermined threshold is present by checking thenot-copied use frequency in the file system configuration information(S26). Because data whose not-copied use frequency exceeds thepredetermined threshold is not present at the early stage of datamigration (S26: NO), data is copied to the migration destination storageregion 34 in a predetermined order beginning with data (files) that havenot yet been copied (S29). Data waiting to be copied can be grasped bychecking the “status” in the file system configuration information. Withrespect to data for which copying has started from the “not copied”status, the “being copied” status is given thereto and registered in thefile system configuration information (S30). When copying is completed,the status of the copied data is changed from “being copied” to “copied”(S31). The processing of S25 to S31 is repeated until copying iscompleted with respect to all of the designated migration target data(S32). When all of the migration target data has migrated (S32: YES),data migration processing ends.

In cases where there are no or few access requests from the clients 10during data migration, only the processing of S25, S26, S29, S30, S31and S32 is repeated. However, in a case where a relatively large numberof access requests have arisen from the clients 10 during datamigration, the not-copied use frequency of the data waiting to be copiedis counted up each time that data is requested by the clients 10. In acase where, for example, access requests from the clients 10 are evenlydispersed in data groups of the “not copied” status and none of thenot-copied use frequencies of the data exceeds the predeterminedthreshold, priority copying is not initiated.

However, in a case where the access requests from the clients 10 areconcentrated in regard to specific data, the not-copied use frequency ofthat data exceeds the predetermined threshold (S26: YES). In a casewhere the status of data having a large access demand is “not copied”(S27: YES), the initial order is superceded and that data is placed atthe top of a queue so that it is given priority and copied to apredetermined position in the migration destination storage region 34beginning with data having a high not-copied use frequency (S28). Then,the not-copied use frequency of the data for which priority copying hasbeen designated is reset (S28). While the data for which prioritycopying has been designated is being copied, the status of that data ischanged to “being copied” (S30). In a case where copying has beencompleted, the status of that data is changed to “copied” (S31).

A case where data whose not-copied use frequency exceeds thepredetermined threshold has a status other than “not copied” (S27: NO)is a case where data with a high use frequency is of the status“accessing migration source”. That is, because the use frequency ishigh, it is situation where the data is being continuously accessed bythe clients 10 without there being time to copy the data. In this case,the data is successively migrated to the migration destination storageregion 34 in accordance with the initial order beginning with data ofthe “not copied” status (S29).

In a case where access to the same data is continually requested by thesame client 10, the not-copied use frequency can be counted up or thesystem can be configured to not accept continuous access from the sameclient 10.

FIG. 7 is a flow chart showing the details of the client access controlprocessing (S24) of FIG. 6. When access is requested by any of theclients 10 during data migration, the migration destination NAS 30checks the file system configuration information (S41). The migrationdestination NAS 30 determines whether the status of the data to whichaccess has been requested by the client 10 corresponds to “not copied”,“being copied”, “copied” or “accessing migration source” (S42).Processing thereafter differs in correspondence to the status of thedata.

In a case where the status of the data to which access has beenrequested by the client 10 is “not copied”, the migration source NAS 30changes the status of that data from “not copied” to “accessingmigration source” (S43), as shown in FIG. 7. The migration destinationNAS 30 also increments the value of the not-copied use frequency of therequested data by 1 (S44). Then, the migration destination NAS 30provides the data requested by the client 10 in a read/write mode fromthe migration source storage region 33 (S45). The migration destinationNAS 30 monitors whether or not access by the client 10 has beencompleted (S46). In a case where access by the client 10 has beencompleted (S46: YES), the migration destination NAS 30 determineswhether or not access to the same data is being requested by anotherclient 10 (S47). In a case where an access request from another client10 has not arisen (S47: NO), the migration destination NAS 30 returnsthe status of the data from “accessing migration source” to “not copied”and ends the processing (S48). In a case where an access request fromanother client 10 has arisen (S47: YES) the migration source NAS 30 endsthe processing with the status of the data remaining set to “accessingmigration source”.

It should be noted that, although description in regard to access rightsmanagement is omitted from the drawings, the migration destination NAS30 conducts access rights management on the basis of the userinformation 231 inherited from the management source NAS 20. Thus,clients 10 that do not have access rights cannot access data whereaccess control has been set. Also, in S45, a case was described wherethe data was provided from the migration source storage region 33 to theclient 10 (having an access right) in the read/write mode. However, in acase where, for example, the access right of the client 10 is read-only,the data is supplied in a read-only mode.

Next, client access processing in a case where the status of the data towhich access has been requested by the client 10 is “accessing migrationsource” will be described. As shown in the flow chart of FIG. 8, in acase where access to data (whose status is “accessing migration source”)that is being accessed by a certain client 10 is requested by anotherclient 10, the migration destination NAS 30 increments the not-copieduse frequency by 1 (S51).

Additionally, the migration destination NAS 30 provides the client 10with the requested data from the migration source storage region 33 inthe read-only mode (S52). The reason the data is presented in theread-only mode is in order to prevent mismatching of the data becausethe data is being used by another client 10 (first client) that hadpreviously accessed that data. The migration destination NAS 30 monitorswhether or not access by the client 10 (second client) has beencompleted (S53). In a case where access by the second client 10 has beencompleted (S53: YES), the migration destination NAS 30 determineswhether or not access to the same data is being requested by yet anotherclient 10 (third client) (S54). In a case where an access request fromthe third client 10 has not arisen (S54: NO), the migration destinationNAS 30 returns the status of the data from “accessing migration source”to “not copied” and ends the processing (S55). In a case where an accessrequest from the third client 10 has arisen (S54: YES), the migrationdestination NAS 30 ends processing with the status of the data remainingas “accessing migration source”. The not-copied use frequency of thedata to which access has been continuously requested by plural clients10 is increased in increments of 1 and exceeds the predeterminedthreshold. Data having a not-copied use frequency exceeding thepredetermined threshold is given priority over other not-copied data andcopied from the migration source storage region 33 to the migrationdestination storage region 34.

Client access processing in a case where the status of the data to whichaccess has been requested by the client 10 is “being copied” will bedescribed on the basis of FIG. 9. In a case where access to data havingthe “being copied” status has been requested by the clients 10, themigration destination NAS 30 provides the requested data from themigration source storage region 33 to the client 10 in the read-onlymode (S61). The reason the data is presented in the read-only mode is toprevent mismatching of the data. The migration destination NAS 30monitors whether or not access by the client 10 has been completed(S62), and ends the processing when access has been completed (S62:YES).

Client access processing in a case where the status of the data to whichaccess has been requested by the client 10 is “copied” will be describedon the basis of FIG. 10. In this case, the data requested by the client10 is already stored in a predetermined place in the migrationdestination storage region 34. Thus, the migration destination NAS 30provides the data requested by the client 10 to the client 10 from themigration destination storage region 34 and ends the processing (S71).

A specific example of the above data migration method will be describedon the basis of FIG. 11. FIG. 11 is a schematic diagram showing therelation between the migration source NAS 20, the migration destinationNAS 30 and the client 10. In the following description, the migrationsource NAS 20 is disposed with plural shared file systems “/mnt1” and“/mnt2”. Thus, the migration destination NAS 30 is also disposed withplural shared file systems. Each shared file system can be configured toprovide services according to respectively different file sharingprotocols. Depending on the case, it is also possible to consolidate theplural shared file systems that the migration source NAS 20 has into oneshared file system in the migration destination NAS 30. In this case, itis necessary to notify the client 10 of the change in the file sharingenvironment before the file sharing service is provided from themigration destination NAS 30.

The host name of the migration source NAS 20 prior to data migration is“FS1” and the IP address is “192.168.100.100”. At a preliminary stagebefore data migration begins, the network environment information of themigration source NAS 20 is changed so that the host name is changed from“FS1” to “FS1SRC” and the IP address is changed from “192.169.100.100”to “192.168.100.200”. The migration source NAS 20 restarts with thenetwork environment information designated by the migration destinationNAS 30 (S100). Thus, only the migration destination NAS 30 perceives thepresence of the migration source NAS 20.

When the migration destination NAS 30 confirms that the networkenvironment information of the migration source NAS 20 has beenswitched, the migration destination NAS 30 switches its own parametersto the parameters that had been initially set in the migration sourceNAS 20. Assuming that the host name of the migration destination NAS 30at the time of temporary launch is “FS1 target” and the IP address is“192.168.10”, the migration destination NAS 30 changes the host namefrom “FS=b 1target” to “FS1” and changes the IP address from“192.168.10” to “192.168.100.100”. Thus, the migration destination NAS30 is recognized on the communications network as the migration sourceNAS 20.

Prior to data migration, the client 10 mounts the shared file system“/mnt1” whose host is “FS1” on the mount point “fs1mnt”. That is, priorto migration, the client 10 network-mounts and uses the shared filesystem “/mnt1” that the migration source NAS 20 has (S102).

The migration destination NAS 30 network-mounts the shared file systems21A and 21B that the migration source NAS 20 has on the basis of theuser information inherited from the migration source NAS 20 (S103). Inthis case, the user account “admin” for exercising the management rightis used in the migration source NAS 20. In the drawing, the shared filesystem 21A (“/mnt1”) of the migration source NAS 20 is mounted on themount point “/mnt1src”, and the shared file system 21B (“/mnt2”) of themigration source NAS 20 is mounted on the mount point “/mnt2src”.

The migration destination NAS 30 copies the data of the shared filesystem mounted in the migration source storage region 33 to the sharedfile system of the migration destination storage region 34, whereby thedata migrates (S104). The name of the migration destination shared filesystem is set to be the same as the name of the migration source sharedfile system. Thus, when the shared file system is accessed, theconnection destination of the client 10 becomes the migrationdestination NAS 30 with the client 10 maintaining its prior setting(S105) That is, the client 10 network-mounts the shared file system ofthe migration destination NAS 30 instead of the shared file system ofthe migration source NAS 20 without being cognizant of this.

FIG. 12 is a schematic diagram showing a state where data migrationprocessing and client access processing are executed in parallel. Themigration destination NAS 30 uses the copy status management function 54to monitor and grasp, per each piece of data (per file), the migrationstatus of the data from the migration source storage region 33 to themigration destination storage region 34 (S110). It will be assumed that,during data migration processing, a request from the client 10 foraccess to the network-mounted migration source shared file system hasarisen in the client 10 (S111). The access request from the client 10 istransmitted to the migration destination NAS 30 rather than to themigration source NAS 20 (S112).

The migration destination NAS 30 uses the client access control function55 to process the access request from the client 10 (S113). The clientaccess control function 55 references the data migration status graspedby the copy status management function 54 to confirm the migrationstatus of the data to which access has been requested by the client 10.In a case where the data requested by the client 10 has alreadymigrated, the client access control function 55 provides the data to theclient 10 from the migration destination storage region 34. In a casewhere the data requested by the client 10 has not yet migrated, theclient access control function 55 provides the data to the client 10from the migration source storage region 33.

FIG. 13 is a schematic diagram showing a state where the shared filesystem provided to the client 10 is switched by the migration situationof the data. It will be assumed that the client 10 has an access rightfor accessing the data. It will also be assumed that the migrationsource NAS 20 has two shared file systems “/mnt1” and “/mnt2”, that a“file1” and a “file2” are stored in “/mnt1”, and that a “file3” and a“file4” are stored in “/mnt2”.

The shared file systems of the migration source NAS 20 arenetwork-mounted in the migration source storage region 33 of themigration source NAS 30. “/mnt1” is mounted on the mount point“/mnt1src” and “/mnt2” is mounted on the mount point “/mnt2src”. Twoshared file systems “/mnt1” and “/mnt2”, which respectively correspondto the shared file systems of the migration source NAS 20, are formed inthe migration destination storage region 34 of the migration destinationNAS 30.

Here, it will be assumed that “file1” has already been copied from themigration source storage region 33 to the migration destination storageregion 34 (“copied” status). It will also be assumed that “file2” isbeing copied from the migration source storage region 33 to themigration destination storage region 34 (“being copied” status). It willalso be assumed that “file3” has not yet been copied from the migrationsource storage region 33 to the migration destination storage region 34(“not copied” status). And it will be assumed that “file4” has not yetbeen copied from the migration source storage region 33 to the migrationdestination storage region 34 and is being accessed by a certain client10 (“accessing migration source” status). When the client 10 requestsaccess to “file1” to “file4” under these conditions, “file1” is providedto the client 10 from the migration destination storage region 34 in theread/write mode (R/W mode) because it has already been copied. In a casewhere the client 10 requests access to “file2”, “file2” is provided tothe client 10 from the migration source storage region 33 in theread-only mode (R mode) because it is being copied. In a case where theclient 10 requests access to “file3”, “file3” is provided to the client10 from the migration source storage region 33 in the read/write modebecause it is not present in the migration destination storage region34. In a case where the client 10 accesses “file4”, “file4” is providedto the client 10 from the migration source storage region 33 in theread-only mode because it is being accessed by another client 10 in themigration source storage region 33.

The present embodiment is configured in this manner and can processaccess from the clients 10 while causing the data to migrate from theexisting NAS 20 to the new NAS 30.

Also, in the present embodiment, because data migration processing andclient access processing can be independently executed in parallel, thefile sharing service can be provided to the clients 10 with a relativelyhigh responsiveness even during the data migration processing. Inparticular, because data that has not yet been copied to the migrationdestination NAS 30 is provided to the client 10 from the migrationsource NAS 20 rather than being provided to the client 10 after beingcopied to the migration destination NAS 30, the responsiveness can beraised.

Also, because data migration processing is initiated after the userinformation and the system environment information have been inherited,access from the clients 10 can be processed even during data migration.

Moreover, in the present embodiment, because the demand with respect todata that has not yet been copied to the migration destination NAS 30 isinvestigated so that data with a high access frequency is given priorityand caused to migrate, data can be rapidly migrated to the migrationdestination NAS 30 beginning with data whose actual demand is large.Thus, data whose use frequency is high can be rapidly provided from themigration destination NAS 30.

Also, in the present embodiment, because the network environmentinformation of the migration source NAS 20 is switched and the migrationdestination NAS 30 inherits the network environment information that hadbeen initially set in the migration source NAS 20, the migrationdestination NAS 30 can be disguised as if it were the migration sourceNAS 20 without changing the physical network configuration. Thus, theclients 10 can access the shared file system with their prior settingswithout having to change any settings.

Moreover, in the present embodiment, because selection of the sharedfile system provided to the clients 10 and selection of the method ofprovision (read/write mode or read-only mode) are conducted incorrespondence to the four types of data migration status (the “notcopied” status, the “being copied” status, the “copied” status and the“accessing migration source” status), data can be appropriately providedin correspondence to the migration status of the data.

Also, in the present embodiment, because whether or not the migrationsource NAS 20 has restarted is monitored on the basis of new networkenvironment information and the network environment information of themigration destination NAS 30 is updated after it has been confirmed thatthe migration source NAS 20 has restarted, adverse affects accompanyingthe switching of the network environment information can be prevented.

It should be noted that the present invention is not limited to theabove-described embodiment. A person skilled in the art can make variousadditions or changes within the scope of the invention.

1. A virtual file system, comprising: a first file server; a second fileserver; and a network through which at least the first file server isconnected to a client; wherein the first file server includes a controlunit, a first shared file system and a virtual shared file system, andthe control unit is operatively connected to receive access to a file ofthe virtual shared file system and to relay the file access of thevirtual shared file system to a file in a second shared file system ofthe second file access server.
 2. A virtual file system according toclaim 1, wherein the control unit is further operatively connected toreceive access to a file of the first shared file system of the firstfile server.
 3. A virtual file system according to claim 1, wherein thecontrol unit includes a table having data on the virtual shared filesystem, the first shared file system and the second shared file system.4. A virtual file system according to claim 1, wherein the control unitrelays the file of the virtual shared file system in a read-write mode.5. A virtual file system according to claim 1, wherein the control unitrelays the file of the virtual shared file system in a read-only mode.6. A virtual file system according to claim 1, wherein the virtualshared file system is a plurality of virtual shared file systems.
 7. Avirtual file system according to claim 6, wherein the second shared filesystem is a plurality of shared file systems corresponding to theplurality of virtual shared file systems.
 8. A virtual file systemaccording to claim 1, wherein the client is operatively connected to usea first mountpoint to access the virtual shared file system, and thefirst server is operatively connected to use a second mountpointrelating to the first mountpoint to access the second shared filesystem.
 9. A virtual file system according to claim 8, wherein the firstmountpoint is configured with a name different from a name of the secondmountpoint.
 10. A virtual file system according to claim 8, wherein thecontrol unit includes data on the relationship between the virtualshared file system and the second mountpoint.
 11. A virtual file systemaccording to claim 8, wherein the second mountpoint is determined by auser account.
 12. A method for operating a virtual file system thatincludes a first file server, a second file server and a network throughwhich at least the first file server is connected to a client, whereinthe first file server includes a control unit, a first shared filesystem and a virtual shared file system, the method comprising the stepsof: receiving at the control unit access to a file of the virtual sharedfile system; and relaying the file access of the virtual shared filesystem to a file in a second shared file system of the second fileserver.
 13. A virtual file system according to claim 12, furthercomprising the step of: receiving at the control unit access to a fileof the first shared file system of the first file server.
 14. A virtualfile system according to claim 12, wherein the control unit includes atable having data on the virtual shared file system, the first sharedfile system and the second shared file system.
 15. A virtual file systemaccording to claim 12, wherein the step of relaying the file of thevirtual shared file system to a file in a second shared file system ofthe second file server includes placing the control unit in a read-writemode.
 16. A virtual file system according to claim 12, wherein the stepof relaying the file of the virtual shared file system to a file in asecond shared file system of the second file server includes placing thecontrol unit in a read-only mode.
 17. A virtual file system according toclaim 12, wherein the virtual shared file system is a plurality ofvirtual shared file systems.
 18. A virtual file system according toclaim 17, wherein the second shared file system is a plurality of sharedfile systems corresponding to the plurality of virtual shared filesystems.
 19. A virtual file system according to claim 12, furthercomprising the steps of: using a first mountpoint for accessing thevirtual shared file system by the client; and using a second mountpointfor accessing the second shared file system by the first server, whereinthe second mountpoint is related to the first mountpoint.
 20. A virtualfile system according to claim 19, wherein the first mountpoint isconfigured with a name different from a name of the second mountpoint.21. A virtual file system according to claim 19, wherein the controlunit has data on the relationship between the virtual shared file systemand the second mountpoint.
 22. A virtual file system according to claim19, wherein the second mountpoint is determined by a user account.